This invention relates to a system for detecting the spatial relationship between a crystalline body and the processing reference direction of a processing device so that the crystalline body may be positioned at a desired location and orientation relative to the machine to facilitate processing.
In the manufacture of semiconductor wafers, the wafers are cut from single crystal ingots. Typically these ingots are irregular in shape. To facilitate subsequent processing, the ingot is normally shaped by removing material from its surface to leave an accurately shaped ingot, such as a cylinder.
The ingot shaping step is typically performed on commercially available machines such as the grinding machine manufactured by Ueda Giken Co. of Japan and available in the United States through Cybeq Systems of Menlo Park, Calif. The Ueda grinding machine has a reference processing direction. The machine has means for holding the ingot with the ingot intersected by the processing reference direction and means for rotating the ingot about the processing reference direction. A tangentially oriented cup-shaped grinding wheel passes along the processing direction removing material from the surface of the ingot while the ingot is rotated. Ingots in the shape of accurate cylinders are obtained.
The cylindrical ingot obtained is then sliced into wafers with specific crystallographic orientations. Typically, it is desirable for the wafers to have &lt;100&gt; or &lt;111&gt; orientations, that is, the planar surfaces of the wafers are substantially normal to their crystallographic &lt;100&gt; or &lt;111&gt; directions. For this reason, it is necessary that the ingot is shaped so that the axis of the cylinder is along one of the &lt;100&gt; or &lt;111&gt; directions.
It is known that, under proper conditions, an ingot may be sandblasted or chemically etched, leaving an ingot surface composed of microscopic crystallographic facets. For Gallium Arsenide ingots, for example, microscopic facets will form on the {110} planes. Light reflected by these microscopic facets will be specularly reflected.
Similar preferential cleavage properties have been used by Hewlett Packard Company to determine the crystallographic orientation of a wafer sliced from an ingot. A single wafer is sliced from the ingot, the wafer cleaved exposing {110} planes, the wafer edge illuminated by a LASER beam, and the reflections of the LASER from the wafer edge observed to determine the crystallographic orientation of the wafer surface. Adjustments are then made to the saw in subsequent slicing operations. To allow an operator to translate the mis-orientation angles into saw adjustments, it is necessary to develop techniques and translation graphs to aid the translation of the mis-orientation angles into saw adjustments.
Other conventional optical devices have been used to determine the orientation of a crystal. One such optical device is the Model 210 Optical Orientation Instrument available from South Bay Technology, Inc. of Temple City, Calif. Model 210 uses a LASER beam to determine the orientation of a crystal. The LASER beam is reflected off a cleaved or preferentially etched crystal surface back onto a target that is perpendicular to the incident LASER beam.
After the orientation of an ingot is determined using model 210, it is necessary to record the crystallographic orientation of the ingot in reference to a known reference frame and then use the information recorded to position the ingot with its selected crystallographic axis substantially coinciding with the processing direction of the grinding machine. In conventional systems, this requires sophisticated and expensive instruments such as goniometers and highly skilled personnel to operate the instruments.
None of the above described conventional systems for placing a crystalline ingot in the desired position for processing are entirely satisfactory. It is therefore desirable to provide improved systems whereby the above-described difficulties are alleviated.